Field of the Invention
The present invention relates to a synthetic lubricant for lubricating a thin film and a magnetic recording medium and, more particularly, to a synthetic lubricant which is capable of firmly adhering to the surface of a substrate of metal, glass, ceramic, carbon or the like in the form of a thin film and providing it with good lubrication and a magnetic recording medium with a lubricant layer formed thereof by using such a lubricant.
The surface of a magnetic recording medium is coated with a lubricant, because lubrication is required between the magnetic recording medium and a magnetic head which come into contact with each other.
In the case of a thin film magnetic recording medium having a high recording density, a flying head is used as the magnetic head. The flying head is operated in the following manner. The flying head is pressed against the medium by the spring force of a load arm and a gimbal at a force of about 10 gf while the medium is not rotated. When the recording medium starts to rotate, the slider portion (made of MnZn ferrite, Al.sub.2 O.sub.3 -TiC, ZrO.sub.2 or CaTiO.sub.3) of the flying head slides on the medium, and when the rotation of the medium assumes a normal state (e.g., 3,600 rpm), the flying head floats at a height of 0.15 to 0.5 .mu.m from the medium on a stream of air which moves together with the rotation of the medium. At this time, the flying head sometimes comes into contact with the surface of the medium due to the vibration applied from the outside or the unevenness of the surface of the medium. When the rotation of the medium is stopped, the flying head gradually approaches the medium and moves on the medium as if to be dragged or stops on the medium while bouncing thereon.
In order to improve the electromagnetic transducing characteristics between a magnetic recording medium and a flying head, it is desirable that the lubricant film applied to the surface of the medium is as thin as possible. When the lubricant film is several hundred .ANG. thick, the flying head adheres to the surface of the medium, in other words, a sticking phenomenon is produced, thereby making it impossible to start to rotate the medium. Thus, it is desirable that the thickness of the lubricant on the surface of the medium is one to several molecular layers.
In order to resist the shock or wear of a flying head by means of the lubricant layer of one to several molecular layers, the lubricant is required not only to be excellent in lubricating properties but also to be firmly absorbed to the metal or carbon of the substrate of the medium.
As the lubricant, perfluoropolyethers are conventionally used and are commercially available as Fomblin produced by Montedison, Italy and Krytox produced by Du Pont, U.S.A. These lubricants are disclosed in U.S. P. Nos. 3,242,218, 3,665,041, 3,715,378, etc.
Perfluoroplyethers are high-quality lubricating oils having excellent thermal stability, heat resistance and resistance to chemicals and low evaporation rate. However, they have very low adsorptivity, and when they are applied to the surface of metal, glass, ceramic, carbon or the like into a thickness of one to several molecular layers as a thin film lubricant, they cannot be firmly adsorbed to the surface of metal or the like. They are therefore limited as a lubricant for the above-described magnetic recording medium.
To improve the adsorptivity of a perfluoropolyether lubricant, attempts have been made on firmly adsorbing it to a magnetic recording medium by attaching a polar group to the end of the perfluoropolyoxyalkylene group and increasing the dipole moment of the polar group to a predetermined value or more. These proposals are disclosed in, for example, U.S.P. Nos. 4,267,238 and 4,268,556.
In a lubricant having a perfluoropolyoxyalkylene group with a polar group attached to the end thereof, the lubricant is adsorbed to metal or carbon by the polar group and the perfluoropolyoxyalkylene group extending from the polar group provides lubricating properties.
However, such a lubricant cannot satisfy the characteristics required when it is used as a lubricant for a thin film magnetic recording medium because the adsorptivity provided solely by the polar group is insufficient for the following reasons:
(1) A thin film magnetic recording medium is used in a very severe state in which the shock and sliding friction of a magnetic head are applied to the medium.
(2) The surface of the thin film magnetic recording medium is a thin film metal medium layer (e.g., Co-Ni layer) formed by sputtering or plating, or a protective film of carbon, ceramic or the like formed on the thin film metal medium layer. The bonding by the polar group cannot maintain sufficient strength between the lubricant layer and a film of such an inorganic material as metal, carbon and ceramic.
These facts bring about the following problems: the lubricant film formed on the surface of the magnetic recording medium peels off, or the molecules of the lubricant film are flown about or blown to the outer peripheral portion of the medium locally by the centrifugal force applied by the rotation of the magnetic recording medium (the characteristic of a lubricant film which cause these phenomena will be referred to as "migrating characteristic"), thereby making it impossible to maintain the lubricant thin film in a uniform state.